1. Field of Invention
The present invention pertains to the casting of ingot. More particularly, this invention is directed to retardation of cooling during the initial stages of the continuous casting of an aluminum ingot in order to optimize shrinkage and minimize distortion in the butt or start end of the ingot.
2. Description of the Art
Traditionally, continuous casting of light metal ingot in the vertical or horizontal direction has followed the practice of introducing molten metal into one end of an open-ended mold. During pouring, the molten metal temperature is preferably held substantially constant to maximize casting efficiency. Typically, the casting mold is relatively short in the axial direction and is hollow or otherwise adapted to receive a liquid cooling medium, such as water, directly against the exterior of the mold. Molds are preferably constructed of aluminum but may also be copper or bronze, all of which exhibit high thermal conductivity. Throughout the casting operation, the cooling medium is applied against the mold in a sufficient amount to extract heat from the molten metal adjacent the mold wall to effectuate at least partial solidification of the molten metal therein. Such cooling produces solidified peripheral portions of an ingot having sufficient mechanical strength and thickness to support a molten phase or generally wedgeshaped crater within the ingot as the ingot is continuously advanced from the exit end of the mold.
At the initiation of the vertical casting operation when molten metal is first introduced into the mold, the bottom or exit end of the mold is closed by a vertically movable bottom block. The ingot is advanced downwardly through the exit end of the mold by moving the bottom block downwardly. The amount of metal removed from the mold as the ingot is advanced from the exit end of the mold is constantly replaced with molten metal poured into the upper or entrance end of the mold. The metal head, i.e. the axial distance from the meniscus of the molten metal to the exit end of the mold, is preferably held constant throughout the casting operation. Lubricants may also be applied to the inside surfaces of the mold to reduce friction between the mold and the ingot and thereby prevent tearing during emergence of the ingot.
It is also conventional practice to apply a liquid cooling medium directly against the exterior surfaces of the emerging ingot. Such a direct cooling applied against the ingot is of sufficient quantity to eventually solidify the interior molten core of the ingot. Transverse solidification of the ingot is progressive such that complete solidification occurs at some axially removed distance away from the exit end of the mold. The supply of coolant applied directly against the ingot may be integral with or separate from the supply of coolant applied to the mold.
As an ingot begins to emerge from a mold, the exterior surfaces of the ingot are directly subjected to cooling, referred to as direct chill. The bottom block is also cooling the ingot butt axially. The thermal gradient between the mold cooling and the direct cooling is significant. The bottom block also generates a substantial thermal gradient for the first few inches of casting. As a result, the butt end of the emerging ingot is subjected to thermally induced stress and strain. Such rapid cooling of the butt end of the ingot causes geometric changes in the ingot as a result of advanced thermal contraction and shrinkage upon rapid solidification. The most common deformities occurring on the butt end or the initially emerging bottom surface of the ingot are known as butt curl and butt swell.
Butt curl is the term used to describe the rounded contour or shape of the butt or buttom end of a continuously cast ingot, as illustrated in FIG. 2. The extent of curl is determined by measuring the vertical distance between the lower corner of an ingot face and the top edge of the starting block, indicated by dimension A in FIG. 2. Curl is caused by thermally induced strains in the ingot mass that arise as a result of an excessively rapid cooling of the emerging butt end of an ingot. Curl decreases as ingot width approaches ingot thickness, thus square or round ingots do not exhibit much curl. However, ingots having a higher width to thickness ratio exhibit increasing amounts of curl.
Butt curl is a problem primarily because it results in an undesirable amount of end scrap that must be removed from an ingot prior to rolling. Another problem may arise as a result of butt curl, if the rate of curl or inward solidification shrinkage exceeds the casting rate. If the ingot is being cast downwardly at a slow rate, the solidified shell of the ingot may actually be rising upwardly at a faster rate toward the mold in response to the curl. If this upward movement occurs for an extended time period, the hot molten metal in the crater may actually melt through the rising bottom causing a metal breakout. Likewise, if the solidified shell has risen above the mold/metal interface, the shell thickens and shrinks away from the mold leaving a wide gap between the mold and the ingot. Then, as the ingot proceeds downward, the molten metal spills over the solidified edge and rushes outwardly of the mold through the gap between the mold and the ingot. This condition is commonly called a "yo-out". These problems are particularly apparent when casting ingots having high width to thickness ratios. For example, ingots having a width of from approximately 40 to 72 inches (1016 to 1829 mm), and a thickness of from approximately 20 to 26 inches (508 to 660 mm), typically required slow casting speeds during the initial stages of the continuous casting operation.
It has been known that cural can be affected by altering the cooling effect of the direct water. For example, it appears that curl can be reduced by retarding the cooling effect during the first few inches of ingot emergence. One attempt at retarding direct cooling to reduce curl, as disclosed in U.S. Pat. No. 3,441,079, involves pulsing the cooling water from "full on" to "full off" positions for predetermined cycles of time. Such a pulsed water system can require a relatively elaborate pump and valve system to intermittently stop and start the water flow completely and can introduce other complications, especially considering water flow rates sometimes in excess of 300 gallons (1135.5 liters) per minute per mold. If the "full off" time cycle is a pulsed water system is too long, the metal may remelt and perhaps burn through the previously solidified ingot wall.
Butt swell is the term used to describe the undesirable increased thickness of the butt or bottom end of a continuously cast ingot, as illustrative in FIG. 3. Typically ingot molds of rectangular cross section are provided with the longer sidewalls having a pronounced convex curvature. Since solidification shrinkage is greatest near the middle of the longer sidewalls, the convex curvature provided on the ingot sidewalls compensates for such shrinkage. Thus, the convex curvature is practically eliminated after ingot solidification resulting in substantially planar ingot sidewalls on the finally cooled ingot. The exception to the elimination of the convex curvature is at the butt end of the ingot. Since the initial stages of the continuous casting operation employ a relatively slow casting rate, and because the butt end of the ingot lies adjacent a starting block rather than contiguous metal, the initial ingot cooling rate is considerably higher than the cooling rate under stable running conditions. Slow casting rates and rapid solidification at the start of a casting sequence minimize the desirable amount of solidification shrinkage. Therefore, the longer sidewalls retain the bowed configuration provided by the mold until the cooling rate is stabilized, and the casting rate is increased.
Butt swell is a problem because it interferes with normal production handling. Besides causing ingot stacking difficulties, ingots exhibiting butt swell must be subjected to additional conditioning operations prior to rolling. It is common to scalp the entire rolling faces of most ingots. Since scalpers have limited cutting capabilities, it is frequently necessary to remove deformities such as swell before scalping the remainder of the ingot. These additional operations remove excessive metal and require more scalper time, adding to the cost of the ingot.
U.S. Pat. No. 3,933,192 discloses a process for producing continuously cast ingot without butt swell. This disclosed process involves advancing an ingot through a mold having substantially planar sidewalls, then when the casting speed is increased above the low initial speed, the sidewalls of the mold are flexed outwardly. By this process the sidewalls of the butt end of the ingot will conform to the substantially planar sidewalls of the mold, while the sidewalls of the remainder of the ingot, which are cast through a flexed mold, will experience solidification shrinkage and also be generally planar upon final cooling.
Although particularly adapted to vertical casting, the present invention may have utility with regard to horizontal casting. A typical bottleneck in the horizontal direct chill (HDC) continuous casting operation involves running short of available molten metal. When metal runs short, the casting rates may have to be cut back intermittently. During such cutback it is important to retain the same molten crater size and the same head. Since reduced casting rates alone result in increased ingot solidification rates, something must be done with respect to the cooling operation to increase the shrinkage or the ingot will develop a convex shape on the rolling faces. It has been found that uniformly retarding the cooling effect of the direct chill liquid medium in horizontal casting results in maintaining uniformity, with respect to ingot surface contour, during periods of reduced casting rates.
Accordingly, an economical and effective method of uniformly retarding the cooling effect of the liquid medium particularly during continuous casting is desired that will eliminate surface deformities that otherwise occur on the surfaces, particularly the butt end, of a continuously cast ingot.